This invention relates to metal halide arc lamps and, more particularly, to a mercury-free, metal halide arc lamp operating in a range of from 250 to 400 watts.
Present day metal halide arc lamps evolved from pure mercury arc lamps developed earlier this century. The early design consisted of an envelope containing mercury and perhaps a small amount of noble gas to aid in starting. Mercury was originally found to be an ideal arc medium, because it is a liquid having a low vapor pressure at room temperature. Thus, it was easy to strike and sustain an arc. At operating temperatures, mercury becomes completely vaporized, pressure becomes quite high, and the voltage across the lamp increases to the point where efficient power supplies can drive the lamp.
The metal halide lamp or metal halide arc is an improvement to the mercury lamp. In addition to mercury and noble gas, it also contains salts of elements that emit desired radiation. Salts are used because they typically have higher vapor pressures than do the elements themselves. Thus, more of the element reaches the arc stream at a given envelope temperature.
The metal halide arc lamp is more efficient than a pure mercury lamp, because the elements are chosen to emit in the visible region of the spectrum. Also, the salts can be chosen to provide a particular color and color rendition, thus making the metal halide are lamp a most attractive, high performance light source. Designers specify metal halide arc lamps in high power applications, such as streetlights and high bay illumination. However, in present day lighting systems, with improved lamp and system technology, metal halide arcs are used in lower power applications.
Although metal halide arc lamps are superior to pure mercury lamps in efficacy, color, and color rendition, they contain mercury. There are two important reasons for this: (a) the mercury arc lamp is the archetype of arc lamp technology, and has evolved from the earlier, simpler design; and (b) the designer can use the vapor pressure-temperature characteristics of mercury to make lamps that are easy to start and that operate at convenient voltages.
A major disadvantage of lamps that contain mercury is reflected in the fact that mercury is a toxic material that will eventually be disposed of into the environment. Present day manufacturers seek to reduce and/or eliminate mercury from their products whenever possible.
It is, therefore, one of the objectives of the present invention to provide a workable, efficient, metal halide arc lamp that is free of mercury.
It is difficult to design a metal halide arc lamp without mercury. Leaving the mercury out of currently available metal halide arc lamps yields lamps with very low operating voltages. At reasonable currents, the power into these lamps is insufficient to raise the envelope temperature high enough to vaporize the salts. The voltage and the power can be increased by increasing the pressure of the noble gas. However, this makes the lamps difficult, if not impossible, to start.
The present invention reflects the discovery that a mercury-free metal halide arc lamp can be obtained by decreasing the bore size and increasing the arc length. This increases the lamp voltage and the initial power draw. The arc length divided by the bore diameter is herein referred to as the xe2x80x9caspect ratioxe2x80x9d. By way of definition, this application defines lamps with aspect ratios greater than 5 as tubular. The inventors have developed a tubular metal halide arc lamp having an arc length of 80 mm, a bore diameter of 8 mm, and containing a noble gas fill of 100 torr xenon. Initial metal halide arc lamps with this configuration produced starting voltages of 40 to 50 volts. At currents of 5 amperes to 7 amperes, this lamp consumed about 250 watts, which was sufficient to raise the operating temperature of the lamp to a suitable value. Later metal halide arc lamp designs in accordance with this invention were found to operate more efficiently at 400 watts.
It is, therefore, an object of this invention to obviate the disadvantages of the prior art.
It is another object of this invention to provide an improved metal halide arc lamp.
It is yet another object of the invention to provide a metal halide arc lamp that is free of mercury.
Yet another object of the invention is the provision of an environmentally friendly arc lamp.
In accordance with one aspect of the invention, there is provided a mercury-free metal halide arc lamp. The metal halide arc lamp has an envelope of fused silica, an aspect ratio greater than 5, and contain a noble gas such as xenon, argon or krypton and a metal halide. The lamp has fill chemistries comprising iodides of sodium/scandium and iodides of sodium/rare-earth. Sodium, scandium, and various rare earths are known to emit strongly in the visible region of the spectrum. The sodium/scandium molar ratio is varied in a range from about five or six to one, up to eleven to one. The fill chemistries can include cesium. Cesium is known to affect the diameter of the arc, and to some extent the voltage. The lamp operates in a range from approximately 250 to 500 watts.